U.S. Air Force goes vortex surfing to cut fuel consumption
By David Szondy
October 17, 2012
Airplane creating vortices made visible by colored smoke (Image: NASA Langley Research Center )
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The United States Air Force (USAF) is taking flying lessons from geese and spiny lobsters. This may seem like the mother of all bureaucratic errors, but there’s actually some pretty solid science behind it. In exploiting a phenomenon known as “vortex surfing,” the USAF has found that by having C-17 cargo planes flying in formation, it can reduce fuel consumption by up to ten percent.
Vortex surfing involves the whirlwinds of turbulence generated at the wingtips of planes in flight. Because they produce turbulence and drag, aeronautical engineers do their best to minimize these. That’s the reason for the little winglets on aircraft. Now, instead of trying to get rid of vortices, the USAF is trying to exploit them to increase fuel economy.
This may be cutting edge as far as aviation is concerned, but vortex surfing has been around a long time. In fact, nature has been exploiting it for millions of years. Geese flying in formation vortex surf. The lead bird generates vortices as it flies and the other geese position themselves inside the wave of turbulence also known as a slipstream. Though the lead bird is working hard, the vortices it generates increase lift for the following birds. The lead bird has to work harder, but the net energy savings for the flock as a whole goes down.
Lobsters use vortex surfing, too. Spiny lobsters migrate in long queues called “lobster quadrilles” where the principle is the same as with the geese. The lead lobster pushes through the water, stirring up vortices, and the others line up behind to take advantage of the slipstream for an easier march.
Airbus has also proposed flying commercial aircraft in formation in “express skyways” to reduce fuel consumption as part of its “Smarter Skies” initiative that looks to the future of air travel. Now the USAF sees it as a way of decreasing fuel consumption over long-haul cargo flights.
As part of a program called Surfing Aircraft Vortices for Energy, or $AVE, the Air Force carried out tests in September and October at Edwards Air Force Base in California. Two C-17 Globemaster III cargo jets were used to see if what works for a goose will work for a plane big enough to carry an M1 battle tank. The tests involved two aircraft – one flying ahead while the other surfed on the vortex of the leader to gain updraft without expending additional fuel.
Unfortunately, vortex surfing involves more than just clever flying. To carry out the maneuver, the C-17’s autopilot software had to be modified to allow the aircraft to remain in the proper formation on the vortex without pilot assistance.
"The autopilot held the position extremely well – even close to the vortex," said Capt. Zachary Schaffer, commander on one of the test flights. "The flight conditions were very safe; this was as hands-off as any current formation flying we do."
The early tests showed a ten percent fuel savings over the course of the flights. The Air Force said its Air Mobility Command (AMD) conducts 80,000 flights per year, so a ten percent reduction in fuel consumption would translate into millions of dollars in savings. The Air Force Research Laboratory is now analyzing the test data and plans to adapt vortex surfing for other aircraft and missions.
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